The purpose of this multicenter stroke prevention trial (STOP) in sickle cell disease was to identify sickle cell disease children who are at high risk for stroke, using a transcranial Dopler (TCD) sonography, and determine if chronic transfusion prevents stroke in children with abnormal TCD. All potential subjects were screened with TCD. High stroke risk or TCD positive were defined as patients who had a blood velocity greater than 200 cm/sec in one of the vessels screened. Patients were randomized into two treatment arms: chronic transfusion and standard clinical care. Neurological exams and MRIs were performed yearly on all randomized patients to determine if there were significant differences in the incidence of MR brain lesions not associated with clinical symptoms.